The preparation of polyether carbonate polyols by catalytic addition of alkylene oxides (epoxides) and carbon dioxide onto H-functional starter substances (“starters”) has been the subject of intensive study for more than 40 years (Inoue et al., Copolymerization of Carbon Dioxide and Epoxide with Organometallic Compounds; Die Makromolekulare Chemie 130, 210-220, 1969). This reaction is shown in schematic form in scheme (I), where R is an organic radical such as alkyl, alkylaryl or aryl, each of which may also contain heteroatoms, for example O, S, Si etc., and where e and f are integers, and where the product shown here in scheme (I) for the polyether carbonate polyol should merely be understood such that blocks having the structure shown may in principle be present in the polyether carbonate polyol obtained, but the sequence, number and length of the blocks and OH functionality of the starter can vary, and is not limited to the polyether carbonate polyol shown in scheme (I). This reaction (see scheme (I)) is environmentally very advantageous since this reaction constitutes the conversion of a greenhouse gas such as carbon dioxide (CO2) to a polymer. A further product formed is the cyclic carbonate shown in formula (I) (for example, when R=CH3, propylene carbonate).

The formation of copolymers from epoxides (e.g. propylene oxide) and carbon dioxide has long been known. For example, U.S. Pat. No. 4,500,704 describes the copolymerization of carbon dioxide and propylene oxide using DMC catalysts. This process is a batchwise process, meaning that catalyst and the entire amount of propylene oxide are introduced prior to commencement of the reaction and contacted with carbon dioxide prior to heating. However, the charging of the autoclave with the entire amount of epoxide results in the drawback that a large amount of propylene oxide is initially charged, which can lead to the release of about 1400 kJ/kg of polymer in the case of a homopolymerization. Such high amounts of heat can be controlled only with difficulty in a stirred tank and thus entail drawbacks from the point of view of reliable operation.
Batchwise mode in the context of this invention is understood to mean that all the reactants, i.e. epoxide(s), any H-functional starter substance(s) and carbon dioxide, are introduced into the reactor prior to commencement of the reaction. In the context of this invention, semi-batchwise mode is understood to mean that at least one of the aforementioned substances is fed to the reactor over a certain period of time.
WO-A 2006/103213 describes, by way of example, a semi-batchwise process where the H-functional starter substance and the catalyst are initially charged and dried in the initial charge. After the catalyst has been activated by adding a portion of the propylene oxide, further propylene oxide is metered continuously into the reactor and the desired CO2 pressure is set. The continuous addition of propylene oxide takes account of an improved safety concept, among other factors. However, one drawback of this mode of operation in conjunction with an autoclave or stirred tank is that, depending on the reactor volume, the fill height of the reactor, the properties of the liquid phase, the composition of the gas phase and further parameters, a variable amount of carbon dioxide in the reactor arises at a preset start pressure and a preset temperature. This means that, during the reaction, the available amount of carbon dioxide at constant temperature is different depending on the aforementioned parameters. In order to keep the pressure constant, further carbon dioxide has to be metered in, in which case solubility effect also have to be taken into account. Moreover, at the start of the reaction, the amount of catalyst present is high at first and then falls continuously in the course of the reaction. These parameters generally affect the product properties. Moreover, it is to be expected that, in this process concept according to the disclosure of WO-A 2008/092767, DMC catalysts exhibit very poor reaction characteristics, or do not show any reaction, with H-functional starter substances of low molecular weight (for example water, propylene glycol, glycerol). Since starter substances of low molecular weight have an inhibiting effect, these can be used only with difficulty, if at all, particularly in the case of batchwise or semi-batchwise processes where the entire amount of H-functional starter substance is initially charged.
WO-A 2008/092767 discloses a process for preparing polyether carbonate polyols by adding alkylene oxides and carbon dioxide onto H-functional starter substances using DMC catalysts, wherein the reactor is initially charged with one or more starter substances of relatively high molecular weight (e.g. polypropylene oxide of molar mass 460 g/mol), and one or more starter substances of low molecular weight (e.g. monopropylene glycol; molar mass 76 g/mol; see example 1) and alkylene oxide are metered continuously into the reactor during the reaction. According to WO-A 2008/092767, the amount of carbon dioxide incorporated into the polymer is dependent on the CO2 pressure in the reactor, a higher CO2 pressure resulting in higher incorporation of carbon dioxide into the polymer. This has the drawbacks that high-pressure apparatuses that are costly to procure are required for industrial scale preparation of polyether carbonate polyols and, at the same time, a complex safety concept has to be provided because of the relatively large gas volume.
A further drawback of the batchwise processes or semi-batchwise processes disclosed in the prior art is the fact that the catalyst first has to be activated (see, for example, WO-A 2008/092767; example 1), which is disadvantageously associated with an additional step and hence causes additional costs.
A further drawback of a stirred tank is the unfavourably low ratio of surface to volume, the effect of which is that the heat released by the polymerization (>1000 kJ/kg of polymer) can be removed via the surface of the reactor only in a relatively inefficient manner, which can have an adverse effect on the control of the reaction temperature. If the removal of heat is undertaken by means of internal or external heat exchangers, this has the drawback that the procurement of a heat exchanger leads to higher investment in the plant and hence to higher costs. If adequate removal of heat and hence an optimal temperature of the reaction mixture is not established, this can lead to an unfavorable selectivity of the reaction or to loss of catalyst activity. Furthermore, an excessive temperature can result both in destruction of the product and in irreversible deactivation of the catalyst. If the removal of heat in a stirred tank reactor is inadequate, the possible high reaction rate of the highly active DMC catalysts cannot be exploited in full, meaning that there is limitation of the maximum reaction rates because of the limited heat removal performance of these reactor types, and there is generally limitation in this regard of reactors both having internal and having external heat exchangers. The consequence is that the reaction can be run only up to a particular alkylene oxide metering rate at constant temperature, even though a higher alkylene oxide metering rate would be achievable on the basis of the high activity of the DMC catalyst.
A further fundamental drawback of a semi-batchwise or batchwise process is that the process has to be stopped for withdrawal of product, therefore resulting in loss of time.
A continuous reaction of alkylene oxides and CO2 can take place in a backmixed reactor (continuous stirred tank) or in a continuous reactor without backmixing. The continuous reactors with backmixing generally feature the same drawbacks as the stirred tanks operated batchwise or semi-batchwise.
WO-A 2007/135154 discloses a reaction unit having a plurality of laminas A, B arranged one on top of another in parallel, which are microstructured in such a way that each lamina has a multitude of channels arranged in parallel to one another, which form a continuous flow pathway from one side of the plate to the opposite side thereof. It can be used for preparation of polyether polyols from one or more alkylene oxides and optionally carbon dioxide and one or more H-functional starter compounds. In order to enable world scale production, it is necessary to implement “numbering-up”, which leads to uneconomic production because of the complexity of regulation. In the case of use of a suspended heterogeneous catalyst, for example of a DMC catalyst according to the process of the present invention, however, problems are to be expected with regard to the deposition of catalyst in the microreactor and the plates arranged in parallel, which would lead to blockages in a continuously operated process.
EP 1 448 662 B1 discloses a process in which an apparatus composed of a main reactor with a downstream tubular reactor is used as postreactor for preparation of polyether polyols. By contrast, there is no description of the preparation of polyether carbonate polyols.